Output buffers are often used in integrated circuits to convert an input signal with a relatively high resistance to an output signal with a relatively low resistance. For example, output buffers can be used to develop signals representing data on a computer data bus in a microprocessor-based electronic system. When an input signal with a relatively high resistance is converted into an output signal with a relatively low resistance by an output buffer, the output signal generally draws a higher current than the input signal. As a result, the output of an output buffer can be used to drive inputs of other integrated circuit chips, connected, for example, to the computer data bus.
A conventional output buffer circuit 10 is shown in FIG. 1 and includes a switching output buffer 13 with, for example, ten output buffers (10.times.). An output signal 22 of output buffer 13 varies as an input 23 to output buffer 13 varies. The frequency of the variation in output 22 is dependent, at least in part, upon the slew rate of the output, as will be appreciated by those of skill in the art. Output buffer 13 is connected to a ground (V.sub.SS) bus 14 and a voltage supply (V.sub.DD) bus 16. Further, in this example, output buffer 13 drives a capacitance load represented as a capacitor 10C.sub.L.
Conventional output buffer circuit 10 also includes non-switching buffers 15 and 17. As shown, an input 18 of non-switching buffer 15 is tied "low," e.g. tied to ground Vss bus. Hence, an output signal 25 of non-switching buffer 15 is ideally low. Also as shown, with an input 19 of non-switching buffer 17 tied high, e.g. to V.sub.DD bus 16, an output 27 of non-switching buffer 17 is high. It should be appreciated that V.sub.DD bus 16 carries a noisy voltage signal, as a pin inductance, i.e. inductor 32, coupled to a voltage supply V.sub.DD pin 33 creates noise due to current flowing through V.sub.DD bus 16. Similarly, V.sub.SS bus 14 carries a noisy ground signal, due to the fact that a pin inductance, i.e. inductor 34, coupled to a ground supply V.sub.SS pin 35 creates noise due to current flowing through V.sub.SS bus 14. That is, inductors 32 and 34 represent the inductance of bond wires and package pins for voltage supply V.sub.DD pin 33 and ground supply V.sub.SS pin 35, respectively.
In general, non-switching buffers 15 and 17 serve the same purpose as output buffer 13. That is, non-switching buffers 15 and 17 can also drive loads. The loads driven by non-switching buffers 15 and 17 can be loads external to the integrated circuit chip on which output buffer 13 is formed. As such, any noise associated with non-switching buffers 15 and 17 may propagate to other integrated circuits where the noise may result in undesirable consequences. For example, noise superimposed on an otherwise low signal which is intended to prevent an integrated circuit from being reset may be misread as a high signal would result in the inappropriate resetting of the integrated circuit.
Noise associated with output buffer 13 can result when transistors (not shown) of output buffer 13 are switched. While this noise can result from temperature effects, this noise is typically a result of high pin inductance which results when the current flows either from the load 10.sub.CL to the V.sub.SS bus 14, or from the V.sub.DD bus 16 to the load 10.sub.CL. When output signal 22 of output buffer 13 goes low, the noise that results from switching flows through internal transistors (not shown) of non-switching buffer 15, and is manifested in the output signal 25 of non-switching buffer 15 as a departure from the low signal level. This phenomenon is known as "ground bounce." In some cases, the output signal 25 may exhibit "ringing," which is a gradual reduction of the magnitude of ground bounce characteristics.
When output 22 of output buffer 13 goes high, the noise that results from switching transistors associated with output buffer 13 appears in the output signal 27 of non-switching buffer 17 as a departure from the ideal "high" signal level. This phenomenon is referred to as "power-supply droop," or "voltage (V.sub.DD) bounce."
As will be appreciated by those skilled in the art, if output signal 25 of nonswitching buffer 15 goes high due to ground bounce or ringing when output signal 25 is expected to be low, the effect of a high output signal 25 on any circuitry which uses output 25 may have serious consequences. Similarly, if output signal 27 of non-switching buffer 17 goes low when output signal 27 is expected to be high, the low output 27 may affect circuitry which uses output signal 27. As such, what is needed is a method for controlling the noise in an output buffer.